Event Tree Analysis
Master forward-looking accident-sequence analysis: build and prune event trees, quantify sequences, link with fault trees, and place ETA correctly in the ISO 26262 lifecycle.
- Chapters
- 14
- Chapters
- Build Steps
- 6
- Build Steps
- Methods Compared
- 5
- Methods Compared
- Worked Case Study
- 1
- Worked Case Study
- 01The Question the Fault Tree Cannot Answer
- 02Anatomy of an Event Tree
- 03Choosing the Initiating Event
- 04Building the Tree Step by Step
- 05Barriers, Safety Functions and Mitigation
Why it pays for itself
Answer the question FTA cannot
Fault trees explain how a failure happens; event trees show what happens next. Learn the forward-looking method that turns "the barrier failed" into quantified accident sequences with frequencies and severities.
Numbers that survive review
The quantification chapters teach the sequence arithmetic, consistency checks and units discipline - plus the dependency and common-cause corrections that keep a reviewer from dismantling your multiplication in one question.
A method you can actually run
The six-step build method, a full battery thermal event case study, a review checklist and a gallery of nine failure patterns turn ETA from textbook theory into a repeatable project activity.
What you’ll be able to do
Choose Initiating Events That Work
Apply the automotive IE taxonomy, granularity and grouping rules, and the screening funnel to pick initiating events that produce useful trees instead of unmanageable ones.
Build and Prune Event Trees
Construct trees with the six-step method, phrase pivotal events as answerable questions, order barriers by demand sequence, and prune impossible branches with documented rationale.
Quantify Accident Sequences
Compute sequence frequencies from IE frequency and conditional branch probabilities, run consistency checks, aggregate to outcome frequencies, and identify the dominant sequences that drive design.
Link ETA with FTA into Bowties
Attach one fault tree per pivotal event, read sequences as cut sets, and handle dependencies and common cause so the multiplication rule stays honest.
Place ETA in the ISO 26262 Lifecycle
Use the four V-model insertion points, respect the qualitative vs quantitative split, and know when ETA beats or complements FMEA, FTA, HAZOP and STPA.
Turn Outcomes into Risk Statements
Map end states to S classes, plot sequences on a frequency-severity matrix, and use barrier-improvement comparisons to argue design changes with numbers.
Chapter by chapter
- 01
The Question the Fault Tree Cannot Answer
Why forward-looking consequence analysis exists: the inductive vs deductive split in ISO 26262-9 Clause 8, the WASH-1400 origin story, and where automotive teams actually use ETA.
- Inductive vs deductive
- WASH-1400 origins
- Automotive use cases
- 02
Anatomy of an Event Tree
The annotated diagram itself: initiating event, pivotal events as question headers, branch points, sequences and end states, plus drawing conventions and the 2^n growth that pruning keeps in check.
- Pivotal events
- Sequences & end states
- 2^n growth & pruning
- 03
Choosing the Initiating Event
What qualifies as an initiating event: the automotive IE taxonomy, granularity and grouping rules from PRA practice, the sources that feed the list, and the screening funnel that keeps workload sane.
- IE taxonomy
- Granularity rules
- Screening funnel
- 04
Building the Tree Step by Step
The six-step construction method taught on the running example of unintended positive torque, with an interactive stepper and a toggle comparing the pruned tree with the full version.
- Six-step method
- Running torque example
- Pruned vs full tree
- 05
Barriers, Safety Functions and Mitigation
What the pivotal events actually are: barrier taxonomy, the defense-in-depth stack, mapping to ISO 26262 vocabulary, barrier quality attributes, and honest treatment of the human barrier.
- Barrier taxonomy
- Defense in depth
- The human barrier
- 06
Quantifying Sequences
The arithmetic on the running example: sequence frequency as IE frequency times conditional branch probabilities, the column-sum consistency check, units discipline, aggregation and dominant sequences.
- Frequency arithmetic
- Consistency checks
- Dominant sequences
- 07
Linking Event Trees and Fault Trees
The linked fault tree approach with one fault tree per pivotal event, the two PRA modelling styles, the bowtie view, and the cut-set reading of a sequence.
- Linked fault trees
- Bowtie view
- Cut-set reading
- 08
Dependencies and Common Cause
Why the multiplication rule rests on an independence assumption, the automotive ways it breaks, three handling options, and an interactive tree showing what a shared supply rail does to outcomes.
- Independence assumption
- Common cause
- Shared-supply example
- 09
Event Trees in the ISO 26262 Lifecycle
Four insertion points on a V-model lane diagram, the qualitative vs quantitative split imposed by the standard, a method-selection checklist, and the work products ETA feeds.
- V-model insertion points
- Qualitative vs quantitative
- Work products fed
- 10
ETA vs FMEA, FTA, HAZOP and STPA
The complement map: a five-method comparison focused on blind spots, the running example seen through the eyes of each method, and the combined workflow that lets the methods feed each other.
- Five-method comparison
- Blind spots
- Combined workflow
- 11
From Outcomes to Risk
Mapping end states onto S classes, sequences on a frequency-severity matrix, F-N curves with honest framing, controllability support for the HARA, and acceptance frameworks.
- S-class mapping
- Frequency-severity matrix
- F-N curves
- 12
Worked Case Study: Battery Thermal Event
The full method exercised end to end on an internal cell short in an HV battery: IE definition, a four-barrier set, the pruned tree, quantification and findings with a sensitivity tornado.
- Internal cell short
- Four-barrier tree
- Sensitivity tornado
- 13
Process, Tools and Pitfalls
Running ETA in a real project: the inputs to collect before starting, the build-and-update rhythm, an interactive review checklist, tooling categories, and the nine ways trees go wrong.
- Inputs & rhythm
- Review checklist
- Nine failure patterns
- 14
Advanced Topics and Limitations
Where the classical tree stops: dynamic event trees and timing, event sequence diagrams, human reliability analysis, uncertainty propagation, and the SOTIF and AV boundary.
- Dynamic event trees
- Human reliability
- SOTIF boundary
Not just text: the visual toolkit
Tree Build Stepper
Advance the six-step construction of the running unintended-torque tree, and toggle the pruned tree against the full version.
Barrier Stack
The defense-in-depth stack of barrier types, showing what each layer buys and how it maps to ISO 26262 vocabulary.
Shared-Cause Explorer
A two-barrier tree showing what a shared supply rail does to the severe-outcome frequency when independence breaks.
Frequency-Severity Matrix
Accident sequences plotted as dots by frequency and severity, connecting quantified outcomes to risk acceptance.
Battery Thermal Event: An Internal Cell Short in an HV Traction Battery
The full method from anatomy to risk mapping exercised once, end to end, on the classic seed of a thermal runaway - with real barrier logic, quantification and design findings.
- Initiating event defined with discipline: a specific internal cell short with a frequency and a unit, not a vague battery fault
- Four-barrier set in demand order: BMS detection and derating, cell propagation resistance, occupant warning with egress time, external emergency response
- Pruned four-column tree quantified into five sequences and aggregated to three outcome states
- Occupants-endangered frequency traced to the tail path where all four barriers fail
- Sensitivity tornado: a faster independent warning cuts the endangered state fivefold; better BMS detection has the broadest effect across all states
Unlock in course
Who this guide is for
- Safety analysts who know FTA or FMEA and need the consequence-side complement
- Engineers quantifying barrier and safety-function effectiveness for HV battery, powertrain or ADAS items
- Safety managers deciding which analysis method fits which lifecycle phase
- Engineers from nuclear or process industries mapping PRA practice onto ISO 26262
Frequently Asked Questions
Common questions about Event Tree Analysis
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